By Fiona Pryor Entertainment reporter, BBC News

Britney Spears is the latest in a string of stars to enter rehab

Britney Spears, who has been in rehab for the second time in two weeks, is the latest in a long line of celebrities to be admitted to a clinic for treatment.

Last week, it was announced that Robbie Williams had been admitted to a US clinic on his 33rd birthday "for a dependency on prescription drugs".

In January, Jade Goody entered rehab and escaped the public backlash after being accused of racism on Channel 4 show Celebrity Big Brother.

And actress Lindsay Lohan sought treatment after Christmas for "personal health" after concerns grew over her rapid weight loss and heavy partying.

But what happens once a celebrity passes through the doors of a rehab clinic, where the paparazzi lens can no longer follow?

Journalist and author Nick Foulkes spent five weeks in a US rehab clinic to treat his aggressive mood swings and unhappiness.

It was definitely an upsetting thing to go and do and it was not a decision I took lightly - but I'm glad I did it Journalist Nick Foulkes on why he went to rehab

Speaking to BBC Five Live, he likened the venue to a farm and said he was put in a dormitory with three other men.

Once there, his bag was searched and he was forced to hand over magazines, chocolates and his mobile phone.

"I was kept busy from literally dawn until way past dusk," he said.

That involved group therapy, lectures and lots of written homework - and he said the experience was "harrowing, scary and peaceful".

"It was a strange time," he explained. "It was definitely an upsetting thing to go and do, and it was not a decision I took lightly. But I'm glad I did it."

Psychologist Michael Cohen has worked at a variety of clinics, where he specialises in treating drug and alcohol abuse.

The Priory Clinics have become well known for treating celebrities

"Generally, people who have an addiction are experts in plate spinning, and eventually they can't keep all the plates up in the air," he says.

He thinks people who seek treatment generally "benefit" from going into rehab - but believes some only do it for family or friends, which are the "wrong reasons".

But what about the celebrity clientele - is everyone treated the same once they walk through the door?

"Well they should be," Cohen says. "But I don't think they are sometimes. I think it varies on the clinic."

Some believe health may not be the only factor when it comes to celebrities checking into rehab, especially when negative publicity plays a part.

Heat magazine news editor Hannah Perry remembers when supermodel Kate Moss underwent treatment after being photographed allegedly taking drugs.

Jade Goody checked into rehab after Celebrity Big Brother

"For Kate Moss, it was the only thing she could do," she says.

"You could argue it was a publicity stunt and going into rehab was the way she told the world she was sorry.

"The public love someone who holds their hand up and says sorry. She knew exactly what she was doing and she has turned her career around."

With Goody, Ms Perry thinks she may have gone to rehab after Big Brother to get away from the controversy and death threats.

"I think with Jade, everybody could see what a state she was in," she says.

"It's one of the only places you can go to get away from the prying eye, and in there she was safe.